The market for wavelength tunable light sources, primarily tunable laser sources is currently increasing rapidly, and the use of such light sources is nowadays occurring in fields such as telecommunications. Hence, a plurality of different wavelength tunable light sources are well known in the prior art.
One such light source is disclosed in JP-0 60 21 549. This document discloses a tunable semiconductor laser, comprising a semiconductor gain element having a reflective mirrored end, and an external second mirrored end, said mirrored ends together defining a cavity. Within said cavity, two condenser lenses are arranged, and between these, a rotatable filter is arranged. By rotating said filter, the wavelength of the main lasing mode may be altered. However, this configuration comprises a plurality of components, and is therefore quite sensitive to misalignment, and at the same time expensive to manufacture. It is also difficult to make the resonator mechanically and optically stable over a long time interval. Moreover, it is complicated to achieve wavelength tuning without mode hops.
A second prior art device using the same basic principle is disclosed in “OFC '88/Wednesday poster/124 WQ27, Interference-filter tuning of a semiconductor laser in a misalignment-tolerant degenerate external cavity, P. Zorabedian, W. R. Trutna Jr”. However, even if this construction is insensitive to angular misalignment of the external mirror, this type of wavelength tunable light sources has the disadvantages that they have a somewhat complex structure, and that the structure is sensitive to the individual positions of the components, and hence, component position misalignments may cause a degraded function of the light source. As the configuration above, this configuration also comprises a plurality of components, and is hence not only sensitive to component position misalignment, i.e. misalignment occurring due to relative component movements along the optical axis of the system, but is also expensive to manufacture. Moreover, it is complicated to achieve wavelength tuning free of mode hops.
An alternative resonator structure, having a more simple construction, is disclosed in “Conjugate-Concentric laser resonator, R. V. Pole, Journal of the optical society of America, Vol 55, No 3, pages 254-260”. This document discloses a laser resonator being a spherical resonator, in which the active gain medium is placed centrally in the cavity and also acts like a lens so that a conjugate concentric resonator is achieved. However, this light source is not tunable, and is therefore of limited use in modern applications. Moreover, since the lens element coincides with the optical gain element, this configuration may not be used with semiconductor lasers.
Further examples of tunable external cavity configurations are the so-called Littman and Litrow configurations, which are well known for the skilled man. However, both these configurations are sensitive for misalignment, and a more stable configuration is therefore desired. Furthermore, many of the prior art configurations comprises a plurality of components, and hence suffer from drawbacks regarding misalignment losses. Also, due to the presence of several component surfaces within the cavity, the different cavities described above suffer from large resonator losses, resulting in a reduced optical power output.